Optical coherence tomography (OCT) is a rapidly developing high spatial resolution optical imaging modality. It uses low coherence interferometry for depth discrimination in turbid media. OCT creates two and three-dimensional maps of optical backscattering in turbid media. Since different components of biotissues scatter and absorb light differently and light scattering is also dependent on tissue localization and condition, OCT can provide valuable diagnostic information and can be used as a medical diagnostic imaging and therapy guidance tool. OCT can also be used for non-destructive evaluation of materials and composites, when penetration depth and contrast are sufficient.
Another optical imaging modality based on light scattering is laser confocal microscopy. It uses a different physical principle—(tight focusing instead of coherence gating in OCT) to selectively receive light backscattered from only small spatial area and reject diffuse scattering from all other areas. However, a confocal microscopy image is also no more than two or three-dimensional map of optical backscattering.
A combination of OCT and confocal microscopy, known as optical coherence microscopy (OCM), is a powerful tool for enhanced penetration depth, ultrahigh resolution. This technique requires spatial localization of a received backscattering signal, achieved by simultaneous application of coherence gating and tracking focus with high numerical aperture, moving synchronously with the coherence gate.
All known implementations of these optical modalities can exhibit dependence of the absolute optical backscattering signal from time and environmental conditions. A few examples include temperature and pressure parameter dependence of the optical elements and optical radiation sources, as well as aging of abovementioned optical elements. Also, the acquired spatial profiles of optical scattering can be influenced by imperfectness of scanning mechanisms and those mechanisms also can experience aging and dependence from environmental conditions. These spatial profiles can be specific for some materials, or biotissues location, or tissue pathological conditions. Therefore, separation between a “true” scattering spatial profile and artifacts induced by these factors is important.
For some applications, the absolute level of an optical backscattering signal in OCT is a characteristic of a specific material, or medical condition in biotissues and their components. All of the above shows the need for a tool for optical backscattering calibration or characterization. Ideally, such a tool should have stable and known optical absorption and scattering coefficients, with minimal dependence of such coefficients on time and environmental conditions. Homogeneous and isotropic spatial distribution of optical properties of the tool can greatly facilitate reliable measurement of “true” spatial scattering profiles by acquiring a test profile/image from the tool and using this test profile to quantify and correct above mentioned artifacts.
Another example of optical technologies using optical calibration tools with known optical scattering and absorption is spectroscopy, including fluorescent and absorption spectroscopy, and differential absorption optical devices.
Prior art optical calibration tools are known to use solid state calibration material. For example, U.S. Pat. No. 4,047,032 describes using ceramic as a calibration material, which preferably includes alumina for obtaining necessary optical properties. Other examples of calibration tools using solid state calibration material can be found in U.S. Pat. No. 4,322,164 and U.S. Pat. No. 5,305,633. However, solid state calibration material typically has some anisotropy and also it is more difficult to provide a stable optical contact between a solid calibration material and an optical probe. In many cases, the optical probes are hermetically sealed, being intended to be in contact with the sample, including human or animal biotissue or fluid.
It is also common to use suspensions of micro particles, including latex microspheres and intralipid solutions for optical scattering calibration as described in U.S. Pat. No. 4,744,656, in U.S. Pat. No. 5,123,738, or in U.S. Pat. No. 6,615,062. However these suspensions are not stable and exhibit sedimentation and coagulation. They are also expensive.
U.S. Pat. No. 5,741,441 teaches the use of a non-liquid scatter standard which comprises a normal cuvette filled with a clear silicon rubber gel in which effective light scattering amounts of inorganic particles are suspended. The calibration material taught by this patent exhibits rather stable optical properties, but the calibration tool is not convenient for calibrating optical measuring devices intended for use in medical applications.
A calibration tool for an optical measurement device having an optical probe which is more cost-effective and more convenient in medical applications, while exhibiting stable optical properties is therefore desirable and provided by this invention.